1. Field of the Invention
This invention relates to a process for recovering energy from an ethane-rich stream flowing into a pyrolysis furnace, and in particular, to a process for recovering energy from an ethane recycle stream collected from the bottom of an ethylene-ethane splitter.
2. Description of the Prior Art
Ethylene is an important chemical in the petrochemical industry, particularly in the production of polymers. Ethylene may be obtained through separation processes from hydrocarbon mixtures which are derived from various sources such as normal refinery operations, cracking processes and the like. The content of the hydrocarbon mixture containing ethylene will vary depending upon the source of the hydrocarbon mixture. However, it is typical that a hydrocarbon mixture used in an ethylene production plant will include hydrogen, methane, ethane, ethylene and other higher hydrocarbons.
In general, an ethylene plant produces ethylene and by-products by pyrolysis reactions which take place in pyrolysis furnaces. The feedstock used may be ethane, propane, butane, naphtha, heavier hydrocarbons, or any combination of these. The stream leaving the pyrolysis furnace is comprised of ethylene, lighter by-products, heavier by-products and unreacted feedstock. The stream leaving the pyrolysis furnace is cooled compressed then separated into various product streams. The main product stream is the ethylene product. Typical by-product streams are propylene, hydrocarbons, gasoline, fuel oil, fuel gas, hydrogen, ethane and propane. Of these, ethane and propane are valuable as pyrolysis furnace feedstocks, and are usually recycled to the pyrolysis furnaces and cracked to extinction to produce additional ethylene. The section of an ethylene plant in which the various products and by-products are separated is referred to as the fractionation section. The fractionation section almost always includes a demethanizer, deethanizer, and an Ethylene-ethane splitter (C.sub.2 splitter). Other fractionation columns such as a depropanizer, a debutanizer, a propane-propylene splitter and others are often also included. A more detailed description of an ethylene plant, the contents of which are specifically incorporated herein by reference, is set forth in an article entitled "Ethylene from NGL feedstocks" published in the March 1984 edition of "Hydrocarbon Processing", pages 83-88.
The fractionation section of an olefin plant requires refrigeration to obtain fractionation. A demethanizer requires very cold refrigeration for reflux condensation, see, for example, U.S. Pat. Nos. 3,443,388 and 3,902,329, the disclosures of which are incorporated herein by reference. The separation of ethylene from ethane also requires refrigeration for reflux condensation, see, for example, U.S. Pat. No. 3,320,754, the disclosure of which is incorporated herein by reference.
The refrigeration required by the fractionation and the compression sections of an ethylene plant is normally provided by cascade refrigeration system. In such a system, process chilling requirements are provided by two or more different refrigeration fluids, one providing chilling at warmer temperatures, the other providing chilling at colder temperatures. In a typical ethylene plant, a higher temperature refrigerant such as propylene refrigerant provides chilling in the temperature range of ambient to about -45.degree. F., and a lower temperature refrigerant such as ethylene refrigerant provides chilling in the temperature range of about -45.degree. F. to about -150.degree. F. Heat absorbed by the lower temperature refrigerant is removed by the higher temperature refrigerant, while heat absorbed by the higher temperature refrigerant is removed by cooling water or air. The refrigeration energy for each refrigerant fluid is provided by a multi-stage refrigeration compressor.
In a typical ethylene plant, the ethylene-ethane fractionating column, commonly termed the C.sub.2 splitter, is one of the major consumers of refrigeration energy. Refrigeration to this column is supplied either by the higher temperature refrigerant, or by the lower temperature refrigerant. When ethylene is the low-temperature refrigerant, and is used to supply the C.sub.2 splitter refrigeration energy, the ethylene refrigeration system can be integrated with the C.sub.2 splitter overhead system. A more detailed description of a C.sub.2 splitter and its refrigeration system, the contents of which are specifically incorporated herein by reference, is set forth in an article entitled "Ethylene from NGL Feedstocks-Low Pressure C.sub.2 Splitter", published in Jan., 1984 edition of "Hydrocarbon Processing", pages 105-108.
Since the C.sub.2 splitter is a major consumer of refrigeration energy, significant reduction of refrigeration consumption, or recovery of refrigeration energy from this column or its auxiliaries, can significantly lower the consumption of energy in the plant, and the cost of power and refrigeration equipment in an ethylene plant.
The bottom product from the C.sub.2 splitter is the ethane-rich stream which leaves the C.sub.2 splitter as chilled liquid and is recycled to the pyrolysis furnaces. On route to the furnaces, refrigeration energy is recovered from the ethane-rich stream at pressures higher than the pyrolysis furnace inlet pressure. The ethane rich stream is expanded, vaporized and superheated as described below.
The ethane-rich stream leaving the bottom of the C.sub.2 splitter is expanded to a pressure just above the pyrolysis furnaces inlet pressure, usually by expansion across a valve. The expansion causes the ethane-rich stream to chill to its boiling point at a pressure just above the pyrolysis furnace inlet pressure. The ethane-rich stream is then vaporized by chilling a process or refrigeration stream, a process in which most of the refrigeration energy is recovered. The ethane-rich stream may then be superheated, a process in which the rest of its refrigeration energy is recovered, and then flows into the pyrolysis furnace.
A similar refrigeration energy recovery system is employed when a liquid ethane-rich stream from an alternative source is used to feed the pyrolysis furnace. The stream is expanded to a pressure just above the pyrolysis furnace pressure, and then vaporized by chilling a process or refrigeration stream, a process in which most of its refrigeration energy is recovered. It may then be superheated, a process in which the rest of its refrigeration energy is recovered and then flows into the pyrolysis furnace.
Accordingly, enhancing the refrigeration energy recovery from the ethane-rich stream leaving the bottom of a C.sub.2 splitter, as well as enhancing the refrigeration energy recovery from any other liquid ethane-rich streams which are used to feed a pyrolysis furnace, can substantially reduce the cost of refrigeration energy and refrigeration equipment in an ethylene plant.